The present invention concerns a ferroelectric data-processing device, particularly for processing and/or storage of data with active or passive electrical addressing, comprising a data-carrying medium in the form of a thin film of ferroelectric material, wherein the ferroelectric material by an applied electric field may attain a first or a second polarization state by being switched from a disordered state to one of the polarization states or from the first to the second polarization state or vice versa, wherein the ferroelectric material comprises logic elements, wherein a polarization state assigned to a logic element represents a logical value of the logic element, wherein the ferroelectric thin film is provided as a continuous or patterned layer, wherein a first and second electrode structure each comprises substantially mutually parallel strip-like electrodes, such that the electrode structures mutually form a substantially orthogonal x,y matrix, wherein the electrodes in the first electrode structure constitute the columns of the electrode matrix or the x electrodes and the electrodes in the second electrode structure the rows of the electrode matrix or y electrodes, wherein a portion of the ferroelectric thin film at the overlap between an x electrode and a y electrode of the electrode matrix forms a logic element such that the logic elements jointly form an electrically connected passive matrix in the data-processing device.
The present invention also concerns a method for manufacturing the ferroelectric data-processing device, as well as a method for readout in the addressing of logic elements in a ferroelectric data-processing device, particularly a ferroelectric data-processing device according to claims 1-9, wherein the method supports a protocol for readout and comprises steps for respectively reading, verification and reset. Finally the invention concerns the use of a ferroelectric data-processing device according to the invention.
Generally the invention concerns data-processing devices with logic elements implemented in a ferroelectric material. The phenomena of ferroelectricity is in this connection supposed known by persons skilled in the art, as the field is comprehensively treated in the literature, for instance in J. M. Herbert, Ferroelectric Transducers and Sensors, Gordon and Breach, 1982, wherein in pp. 126-130 there is proposed using a ferroelectric memory based on single crystals of barium titanate provided between orthogonal electrodes in an x,y electrode matrix. The author concludes that there are substantial practical difficulties connected with the use of ferroelectric single crystals for information storage in this simple manner. In regard of recent survey literature, reference may be made to R. G. Kepler and R. A. Anderson, Advances in Physics, Vol. 41, No. 1, pp. 1-57 (1992).
To illustrate the development of ferroelectric memories in a historical context, reference may be made to a paper by W. J. Merz and J. R. Anderson titled xe2x80x9cFerroelectric Storage Devicesxe2x80x9d, which was published in September 1955 (Bell Lab. Records, 1:335-342 (1955)) which discloses the use of inorganic ferroelectric crystalline materials, particularly barium titanate in, in memory and switching devices. Particularly they suggest a ferroelectric memory device based on this material, the latter being provided as a planar 50-100 xcexcm thick slab between overlapping sets of parallel electrodes, one set of the electrodes being orthogonal to the electrodes of the other and thus providing ferroelectric memory cells in portions of the ferroelectric material between the overlapping electrodes. Thus they disclose a ferroelectric device with a passive electrode matrix for addressing (see FIG. 10 of their paper), anticipating the general layout of all later ferroelectric memory devices with matrix-based addressing. They even hint at the use of transistors for switching, but forming an active memory cell with a switching transistor and with sufficiently small dimension would hardly be practical before the advent of say integrated field effect transistors.
As mentioned above, the data-processing medium is a ferroelectric material in the form of thin film. Such ferroelectric thin films which either may be inorganic, ceramic materials, polymers or liquid crystals have been known for some time and it may in this connection be referred to the above-mentioned article by Kepler and Anderson. There are for instance from J. F. Scott, Ferroelectric memories, Physics World, February 1995, pp. 46-50, known data storage devices based on ferroelectric memory materials. They all have in common that at least one transistor is necessary in each bit location or memory cell. In the most common embodiments the ferroelectric material is used as a dielectric in the associated memory circuit and comprises a bit-storing capacitor. Due to the high dielectric constant of ferroelectric materials, the capacitor may be made much smaller than otherwise possible and will additionally provide a quite superior charge lifetime. Recently the development has focused on another property of ferroelectric materials, namely their ability to be polarized electrically when they briefly are subjected to a strong electric field. During the polarization process the dipoles of the ferroelectric material attain a preferred orientation, something which results in a macroscopic dipole moment which is retained after the removal of the polarizing field. By thus including the ferroelectric material in the gate electrode structure of a field effect transistor in the memory cell circuit, the transconductance characteristics of the transistors may be controlled by controlling the polarization state of the ferroelectric material. The latter may be switched, for instance by polarizing fields with a direction which either causes a transconductant state xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d in the transistor.
EP patent 0 721 189 discloses a ferroelectric memory with discrete memory cells provided in an electrode matrix. In addition to a discrete ferroelectric capacitor each memory cell also comprises switching means, preferably in the form of at least one transistor. The discrete memory cells hence do not form a passive matrix. With discrete memory cells it shall here be understood that the ferroelectric capacitor is formed by a discrete component, such that the ferroelectric material cannot form a continuous layer in the matrix. There are provided separate data and selection lines and the readout of a stored datum may take place in current or voltage mode on data lines provided for this purpose, but according to a relatively complicated protocol, such as disclosed by patent claim 6. It must also be remarked that the number of memory cells connected in a data signal line must be adjusted in order to accommodate parasitic capacitance on each data signal line during the readout, such that the voltage variation on one of the data signal lines is minimized.
U.S. Pat. No. 5,592,409 concerns a non-volatile ferroelectric memory wherein data may be read out without destruction. The memory cells are included in an active matrix and are formed as transistor structures therein, wherein the gate electrode forms one of the electrodes in a ferroelectric capacitor. It is evident that the ferroelectric capacitors are discrete components. The polarization of the capacitor takes place in a well-known manner, but by the readout which takes place in current mode it is the drain current that is detected, this in order to prevent the stored data from being erased.
Even if the use of ferroelectric materials as mentioned above represents substantial improvements relative to alternative technologies for storage of data, the basic architecture of ferroelectrically based memories is directed to the use of active microcircuits included in each memory cell. This has negative consequences for the achievable data storage density, i.e. the number of bits which may be stored on a given surface area, as well as for the cost of each bit stored, something which partly may be due to complicated manufacturing technology and the use of active semiconductive components.
Recently proposals have been made for a return to ferroelectric memory devices configured as a memory cell array in a passive electrode matrix. Thus U.S. Pat. No. 5,329,485 (U. Isono and al.) discloses a memory element and a matrix memory cell array including memory cells each having a non-linear conductivity bipolar switching element constituted by a multi-layered structure which performs writing/reading operations of a polarization state on a ferroelectric body which forms a recording medium of the memory cell. The switching element is in the form of an insulating film which acts as a switching element to accumulate charges in a charge accumulating ferroelectric capacitor which constitutes the memory cell proper. The insulating film, which particularly may be a polyimid film, allows a direct tunnel current to flow when a voltage exceeding a predetermined value is applied to the insulating film. When the voltage is turned off, the film recovers its insulating property and retains the charges by preventing leakage thereof. According to Isono and al. the film shall have non-linear current voltage characteristics and provides a high write speed without a high operation voltage due to a large part of the drive current of the insulating film being a direct tunnel current. This also allow a high integration density of the memory cells, while the switching film forming a diode junction in the memory cell reduces crosstalk between the cells.
U.S. Pat. No. 5,375,085 discloses another example of a ferroelectric memory in the form of a ferroelectric integrated circuit realized with a passive electrode matrix with a ferroelectric layer provided between the electrode sets forming the substantially orthogonal matrix. As usual, the memory cell is formed in the portion of the ferroelectric layer between the overlapping electrode of each electrode set. By providing an insulating layer over the electrode matrix a second electrode matrix may be deposited on the top of the former and so on, thus forming a stacked structure realizing a volumetric three-dimensional ferroelectric integrated circuit with passive matrix addressing. This is, however, already known from the above-mentioned U.S. Pat. No. 5,329,485, see for instance column 14, 1. 31-36 thereof.
Additionally it could also be mentioned that passive matrix addressing is, of course, well-known in the case of ferroelectric liquid crystal elements as for instance used in liquid crystal displays. Reference may in this regard be made to e.g. U.S. Pat. No. 5,500,749 (Inaba and al.)
It has also been shown that ferroelectric polymer materials may be used in erasable optical memories. For instance there are disclosed devices for ultrafast non-volatile information storage with ferroelectric polymers as the active storage elements (IBM Technical Disclosure Bulletin 37:421-424 (no. 11, (1994)). Preferred embodiments utilize poly(vinylidene fluoride) (PVDF) or PVDF-trifluoroethylene (PVDF-TrFE) copolymers as the ferroelectric material since these polymers can be obtained as very thin films and can have response times of better than 350 picoseconds. The ferroelectric polymers can be used in the gate of a standard dynamic or static RAM device. The most basic information storage device suggested consists of a ferroelectric thin film with a set of parallel conducting electrodes deposited on one side and an orthogonal set of conducting electrodes deposited on the other side. The individual storage cells are formed at the junctures of the opposing electrodes. A stack of two-dimensional passive arrays of this kind can be fabricated by alternately depositing conducting strips and ferroelectric material to build up a three-dimensional array of ferroelectric capacitors which easily could be stacked vertically on an integrated circuit with addressing logic sense amplifiers and thus providing a volumetric or three-dimensional ferroelectric memory.
Further, M. Date and al. has in the paper xe2x80x9cOpto-ferroelectric Memories using Vinylidene Fluoride and Trifluoroethylene Copolymersxe2x80x9d, IEEE Trans. Electr. Ins., Vol. 24, No. 3, June 1989, pp. 537-540, proposed a data medium comprising a dye-doped vinylidene fluoride trifluoroethylene copolymer with a thickness of 2 xcexcm, spin deposited on a ITO coated glass plate. The information is written as sequences of positive and negative polarizations generated by irradiating with a focused laser beam with a diameter of about 5 xcexcm in the presence of sign controlling electric fields. The data is read out pyroelectrically by scanning with a laser beam. A carrier/noise ratio of 48 dB has been obtained by using a regularly repeating data train in the form or 0/1-state with a pitch of 20 xcexcm and with the use of a laser power of 12 mW and field strength of 25 MV/m. The reading speed was then 100 mm/s.
A disadvantage shared by all prior art ferroelectric memory devices is that the arrangement of the electrode matrix gives rise to serious fabrication problems when an organic ferroelectric memory medium is used in combination with inorganic, i.e. metallic, electrode strips and inorganic substrates, due to the need for processing the various materials in different thermal regions. Realized as thin film structures both organic materials as well as crystalline inorganic ferroelectric materials have turned out to be thermally incompatible with the temperatures required for processing the other materials of the device.
The object of the present invention is thus to provide a simple logic architecture which may be used for realizing either bistable switches or memory cells in a data-processing device or to provide a purely ferroelectric data storage device which offers the possibility of storing a very high number of bits in an area unit and which at the same time may be produced in simple manner in high volume with low cost, such that the above-mentioned disadvantages of the prior art thin film devices are avoided.
This object and other advantages are achieved according to the invention with a ferroelectric data-processing device which is characterized in that a layer of an electrical isolating material is provided between and adjacent to the electrodes of the first and the second electrode structure, that the ferroelectric thin film is provided in the form of a continuous or patterned layer over the electrode structures on one side thereof, and that the logic elements are formed respectively in a portion of the ferroelectric thin film along the side edges of an y electrode at the overlap between the x electrode and the y electrode; a method for manufacturing a ferroelectric data-processing device characterized by successive steps for depositing a first electrode structure on a substrate, depositing a layer of electrical isolating material over the first electrode structure, depositing a second electrode structure over the isolating layer, removing the isolating layer where it is not covered by the second electrode structure, such that the electrodes in the first electrode structure is exposed except in the overlapping intersections between the electrodes of respectively the first and the second electrode structure, and depositing a ferroelectric thin film in the form of a continuous or patterned layer over the electrode structures; and a method for readout characterized by applying in the reading step a voltage with a determined polarization to a logic element and detecting a charge transfer between the electrodes thereof as an either high or low first current value indicative of a logical value stored in the logic element, applying in the verification step a voltage of the opposite polarity to that of the voltage applied in the reading step and detecting a charge transfer between the electrodes of the logic element as a high second current value, and, in the case the logical value stored in the logic element was destroyed in the reading or the verification step, applying in the step for reset a voltage to the logic element restoring an initial polarization state thereof.
Advantageously, a logic element forms a bistabile switch in a data processor means or a memory cell in a data storage means.
According to a preferred embodiment of the invention the electrode structures and the ferroelectric thin film are provided on a substrate.
According to the invention the ferroelectric thin film is advantageously formed of a ceramic material or a ferroelectric liquid crystal material or a polymer, the polymer preferably being polyvinylidene fluoride, or a copolymer, the copolymer preferably being a vinylidene fluoride/trifluoroethylene copolymer.
In the method for manufacturing the ferroelectric data-processing device it is according to the invention advantageous that the substrate is formed of a crystalline, polycrystalline or amorphous semiconducting material, for instance silicon.
Advantageously, a continuous layer of an electrical isolating material is deposited between the substrate and the first electrode structure before depositing the first electrode structure on the substrate.
In a first embodiment of the method for readout reset is performed after reading without verification by applying a voltage of the opposite polarity to that of the reading voltage only in the case of detecting a high current signal in the reading step.
In a second embodiment of the method for readout reset is performed after reading in conjunction with verification by applying a voltage of the same polarity as that of the reading voltage only in the case of detecting a low current signal in the reading step.
In the method for readout according to the invention it is particularly preferred applying a voltage which between the electrodes of the logic element generates a field strength which is more than twice the coercivity field of the ferroelectric material. Advantageously, the applied voltage is generated as a ramp voltage or a threshold voltage in the reading and/or verification steps. According to the invention it is preferred that the current detection in the reading step takes place either by sampling in the time domain or in a time window dependent on the saturation time constant of the polarization. Advantageously the current detection, particularly in the latter case, takes place by a level comparison.